1. Field of the Invention
This invention relates to fabrication of electronic devices and, in particular, the fabrication procedures utilizing lithographic techniques.
2. Art Background
Trilevel resists--resists especially suitable for the lithographic definition of small features, i.e., features smaller than 2 .mu.m--have been utilized in the formation of lithographic masks and in the formation of electronic devices. In the former case, the resist is generally delineated with a directed electron beam and an underlying metal layer, e.g., a gold containing layer of an X-ray mask, is then etched to produce the desired mask. In the latter case, the resist is delineated with a directed electron beam, or a previously fabricated mask is employed with exposing radiation, e.g., deep UV, X-ray, or near UV, to delineate the resist.
Trilevel resists include an underlying layer generally deposited directly on the substrate being processed. (The substrate in this context is a mask blank or the semiconductor body including, if present, various levels of, for example, metallization, doped semiconductor material, and/or insulators.) Since the substrate typically, at least for device fabrication, does not have a planar surface, this layer is usually deposited with a thickness that is sufficient, despite the underlying irregularity, to present an essentially planar upper surface. An intermediary layer is then formed on this planarizing layer. The composition of the intermediary layer is chosen so that it is etched at least 5 times slower than the planarizing layer by a plasma that is capable of removing the underlying layer. A third layer (an overlying layer) that is delineable by exposure to radiation and by subsequent treatment with a suitable developing medium is formed on the intermediary layer.
The trilevel resist is patterned by first delineating the overlying layer in the desired pattern. This pattern is then transferred to the intermediary layer through conventional techniques such as dry processing, e.g., reactive ion etching, causing an uncovering, in the desired pattern, of the underlying layer. The uncovered regions, generally of organic materials, are then removed with an oxygen plasma. Intermediary layers of materials such as silicon dioxide, that are essentially unaffected by an oxygen plasma, are employed to avoid its destruction during plasma processing and thus degradation of the transferred pattern.
Although the trilevel resist has proven to be an excellent expedient for producing fine features, it does involve several discrete processing steps. Since there is always a desire to reduce processing steps and the associated costs, there has been a significant effort to develop a bilevel system yielding the advantages, i.e., planarization and high resolution, of a trilevel system. Attempts typically have been made to combine the attributes of the intermediary layer and the overlying layer into a single layer. To effect this combination, the resultant layer should be both delineable and also should be at least 5 times more resistant than the underlying layer to the medium utilized to develop the underlying layer.
Other properties that depend on the particular resist application are also desirable for the overlying layer of a bilevel resist. For example, since masks or custom devices formed at least in part by direct writing are typically fabricated by relatively slow electron beam exposure, the relationship of resist tone to mask geometry is often chosen to minimize the area to be exposed and, in turn, to reduce exposure time. Thus, certain mask geometries, those with a majority of transparent area, are more quickly exposed with a negative-acting resist overlying layer--a layer in which the exposed material remains after development. In contrast, certain mask geometries, those with a majority of opaque areas, are more quickly exposed with a positive-acting resist overlying layer--a layer in which the exposed material is removed after development.
For trilevel processing with relatively rapid exposure techniques, e.g., UV exposure, positive resist materials have typically been utilized, at least in part, because they generally afford higher resolution. For example, a resist material described in commonly assigned U.S. Pat. No. 4,481,049, issued Nov. 6, 1984, which is hereby incorporated by reference, has been disclosed for such uses.
Irrespective of the resist tone and delineation technique, to limit exposure time it is generally desirable that the resist have a relatively high sensitivity. For example, it is typically desirable to have sensitivities that require less than 20 .mu.coulombs/cm.sup.2 preferably less than 10 .mu.coulombs/cm.sup.2 for electron beam exposure and less than 500 millijoules/cm.sup.2 for deep UV exposure.